This invention relates to liquid fuel rocket engines in general and, in particular, to a new and useful rocket engine of a secondary flow design for operation in vacuum or almost in vacuum, and comprising, a combustion chamber with a convergent/divergent thrust nozzle part, in particular, with an expansion rate corresponding to ground pressure, a vacuum thrust nozzle part, fuel pumps to pump the liquid fuels, in particular, oxygen and hydrogen, one or more turbines with admission of hot propellant gases to drive the fuel pumps and with the turbine exhaust gases discharging into the open through a secondary flow nozzle and with a small partial amount of a liquid fuel, in particular, hydrogen, flowing through cooling channels in the wall of the vacuum nozzle part.